An electric power system is controlled so that an electric power supplied to this electric power system balances with a power consumption of a load coupled to this electric power system. However, when a system failure of the electric power system occurs and for example an isolated power system is generated in the electric power system, the supply and demand are temporarily unbalanced and thereby the frequency of the electric power system will deviate from a predetermined frequency.
Therefore, as described in JP-A-57-211940, a technique has been invented for adjusting the supply-and-demand unbalance within an isolated power system by shutting off a power source or a load by online control and thus maintaining the frequency.